fix(resolve): ensure exports has precedence over mainFields

[bluwy]

Description

If the exports field returns a file path that ends with .mjs, mainFields will take precendence over it. This breaks how nodejs resolve and blocks svelte's node condition from being used.

Additional context

Feels a bit late to push this, but I tried to make the smallest change possible to fix this bug specifically, which shouldn't break existing logic.

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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